1. Field of the Invention
The present invention relates to an audio signal processing method for reproducing, in an environment, a sound field originally generated in another environment. The present invention also relates to a sound field reproducing system including a recording apparatus configured to record information on a recording medium and an audio signal processing apparatus configured to generate a reproduction audio signal for use to reproduce a sound field in accordance with information recorded on a recording medium.
2. Description of the Related Art
When a content such as a movie content or a music content is played back, it is known to add sound reverberation to enhance presence in reproduced sound.
One known technique to add reverberation is digital reverb. In the digital reverb technique, a large number of delayed signals with a random delay are generated from an original sound and are added together with the original sound. The amplitude of each delayed signal is determined such that the amplitude decreases with the delay time. Delayed signals with large delay times are fed back to achieve sound reverberation with a greater reverberation time. Thus, it is possible to artificially give a reverberation effect to the original sound. However, parameters used to generate the delayed signals are determined based on audibility of a human operator who sets the parameters, and the process of setting the parameters is very complicated and troublesome. Besides, in this technique, the reverberation is artificially generated without consideration of localization of the original sound, and thus this technique does not allow a good sound field to be reproduced.
Another known technique to create a reverberation effect is to measure an impulse response in an actual sound field space and generate reverberation based on the measurement result including spatial information associated with localization of a sound source. A specific example of this technique is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2002-186100.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-186100, for example, a speaker 3 serving as a sound source for measurement (hereinafter such a speaker for measurement will be referred to simply as a measurement speaker) is placed in a measurement environment (a sound field to be measured) 1 such as a hall as shown in FIG. 1. Note that similar notations are used elsewhere in the present description to denote devices, units, signals, etc. for use in the measurement. For example, microphones for use in measurement are denoted by measurement microphones. Similarly, devices, units, signals, etc. for use in reproduction are denoted by adding “reproduction” before names of devices, units, signals, etc. An audio signal such as a TSP (Time Stretched Pulse) signal by which to measure the impulse response is applied to the measurement speaker 3, and a measurement signal (a sound by which to measure the impulse response measurement) output from the measurement speaker 3 is detected by a plurality of measurement microphones 4a to 4p placed at particular positions in the same sound field. For example, as represented by arrows in FIG. 1, the measurement microphone 4a detects a direct sound from the measurement speaker 3 and reflected sounds which originate from the measurement speaker 3 and which reach the measurement microphone 4a after being reflected in the hall used as the measurement environment. Although not shown in the figure, the other measurement microphones 4b, 4c, 4d and so on detect the direct sound and reflected sounds in a similar manner.
By measuring the impulse response including the reverberation based on the audio signals detected by the respective measurement microphones 4a to 4p, it is possible to determine transfer functions from the measurement speaker 3 to the respective measurement microphones 4.
By using these transfer functions, the sound field in the measurement environment shown in FIG. 1 can be reproduced in an environment in which speakers 8a to 8p are placed, as shown in FIG. 3, at positions similar to the positions of the measurement microphones 4 in the measurement environment shown in FIG. 1.
More specifically, if transfer functions from the sound source to respective positions of the measurement microphones 4 are given, audio signals which should be output from the respective speakers 8 placed at the above-described positions can be given by convolutions of an audio signal to be reproduced and the respective transfer functions. If these audio signals are output from the respective speakers 8, a reverberation effect similar to the in the measurement environment shown in FIG. 1 can be obtained in space surrounded by the speakers 8.
This technique allows a sound field to be reproduced with high accuracy, because the transfer functions determined based on the actual measurement are used. This technique is also excellent to obtain good localization of a sound image in the reproduced sound field.
Note that it is important to place the speakers 8a to 8p in the reproduction environment shown in FIG. 3 at positions geometrically similar to the positions of the measurement microphones 4a to 4p in the measurement environment shown in FIG. 1 so that, in a region surrounded by the speakers 8 in the reproduction environment (that is, in a region on the inner side of a closed surface on which the speakers 8 are located), the sound source in the measurement sound field is precisely reproduced at a location corresponding to the location of the original sound source, and thus the sound field in the measurement environment is precisely reproduced.